This invention relates to hydrophilic, high molecular weight nonionic polyurethanes, to their production and their use as the basis for an adhesive and as a stabilizer in emulsion polymerization.
Hydrophilic polyurethanes are acquiring increasing significance in the form of aqueous dispersions. The dispersions may be stabilized by addition of emulsifiers or by the incorporation of hydrophilic groups. Typical hydrophilic groups are ionic groups, more particularly salts of carboxylic acids and alkali metals or amines. However, aqueous dispersions of polyurethanes with a nonionic internal emulsifier are also known.
Thus, EP 0 497 404 A describes an aqueous polyurethane dispersion based on tetramethyl xylene diisocyanate. The polyurethane contains as internal emulsifier a reaction product of alkoxypolyethylene glycol with an anhydride which is then further reacted with an alkylene oxide, an epoxy alcohol or a diglycidyl ether. According to Example 4, a) 133 parts by weight of a reaction product of methoxypolyethylene glycol and trimellitic anhydride, which finally was reacted with propylene oxide, b) 79 parts by weight of polypropylene glycol, c) 40 parts by weight of the ethylene glycol diether of bisphenol A and d) 146 parts by weight of isophorone diisocyanate are mixed and the resulting mixture is kept at 90xc2x0 C. for 3 hours. This is followed by chain extension with water at 45 to 90xc2x0 C. A dispersion containing particles 50 nm in size characterized by high resistance to water is obtained. This dispersion may be used for the production of adhesives and paints.
Aqueous polyurethane dispersions of the type in question have the disadvantage that, even with high solids concentrations, their viscosity is too low for many applications, for example as a wallpaper adhesive. In addition, the adhesive strength and tack of the correspondingly heavily diluted products are far too low or totally non-existent.
K. C. Frisch investigated the thermal and hydrolytic degradation of linear polyurethanes (Journal of Polymer Science, Vol. 11 (1973), pp. 637-648 and pp. 1683-1690). The polyurethanes were prepared from the diisocyanates toluene-2,4- and -2,6-diisocyanate (TDI), m- and p-xylene diisocyanate (XDI) and dicyclohexyl methane-4,4-diisocyanate (HMDI) and from polyoxyethylene glycol with an equivalent weight of 190, 1,485 and 2,955. In some cases, ethylene glycol was also used as chain-extending agent. Polyurethanes were prepared from these components in solvents at around 70xc2x0 C. in the presence of tin octoate as catalyst. The hydrolysis was investigated using a solution of 0.75 g in 25 ml of solvent containing 10% by weight of isopropanol. The use of the polyurethanes thus produced was not discussed.
U.S. Pat. No. 4,079,028 describes a low molecular weight polyurethane as thickener for a latex and other aqueous systems. The polyurethane contains at least three hydrophobic groups with hydrophilic polyoxyalkylene groups in between. Thus, a polyurethane with an average molecular weight (Mw) of 71,000 terminated by dodecyl groups can be prepared, for example, from 200 g of polyethylene glycol, 1.7 g of dodecyl isocyanate and 1.4 g of TDI in toluene at 75xc2x0 C. in the presence of dibutyl tin laurate. The molecular weight is in the range from 10,000 to 200,000. In the case of the linear polyurethanes (see Examples 1 to 102), the degree of polymerization is generally 1 to 4 and at most 18. Table 19 contains reaction products of the three components diol, diisocyanate and a monofunctional compound, such as monoisocyanate or polyhydric alcohols or amines, as a clear limitation with respect to the known linear thickeners of the two components diol and monoisocyanate on the one hand and monoalcohol and diisocyanate on the other hand. Polyethylene glycol, diisocyanates and other polyols are not mentioned. The known linear polyurethanes are used as thickeners, for example even in adhesive dispersions. Their use as adhesives is not mentioned.
The same also applies to U.S. Pat. No. 4,155,892.
EP 0 537 900 describes corresponding polyurethanes as thickeners for non-aqueous systems.
DE 41 37 247 also describes a thickener of a linear polyurethane which is similarly prepared from a difunctional isocyanate, a polyether diol and a monohydric alcohol.
Accordingly, the problem addressed by the present invention was to provide an aqueous system of polyurethanes which have a high viscosity, even at low concentrations, which could compete in price with pastes and which could be mixed with water in any ratio.
The solution provided by the invention is defined in the claims. The invention is essentially based on the provision of a hydrophilic, high molecular weight nonionic polyurethane which is soluble in water and of which the viscosity in water may-be additionally increased by hydrophobic interactions.